1. Field of the Invention
The present invention concerns an apparatus for coating a pipe or a pipe section. The apparatus of the present kind comprises means for applying a layer of at least one polymer material on the surface of the pipe or pipe section at least along a part of the periphery thereof.
The invention also concerns a corresponding method for coating a pipe or pipe section.
2. Description of Related Art
Steel pipes coated with layers of polymers, such as polyethylene or polypropylene, have been used in oil and gas pipelines for a long time. These kinds of pipes are mechanically strong and have good corrosion resistance along the coated part of the pipe. They are generally welded together using the SAW technique (Submerged Arc Welding) to form a pipeline. To facilitate weld-joining of the pipes at the construction site, end portions of the pipe are usually left without coating at the factory. In practice, the pipes are often coated entirely and in a later stage the polymers are stripped or brushed off at the ends of the pipe. This is called cut-back area and the length is defined through the project specification, normally 150 mm on both pipe ends.
Field-welded pipe joints, i.e. girth welds formed at the construction sites, are sensitive to corrosion. For this reason, a coating layer has to be spread upon the girth weld and on the adjacent, uncoated portions of the pipes so as to completely cover the welded joint and to shield it from moisture and water in the ambience. Thus, the applied polymer layer overlaps with a factory coatings of the pipes joined.
Field-joints coatings are conventionally most typically produced by injection moulding or surface fused tape techniques. Also a number of other techniques such as epoxy-coating, PUR-coating, and coating by vulcanized tapes or shrinkable sleeves have been utilized.
WO 2008/132279, which discloses the features of the preambles of the independent claims, discloses another method and apparatus for coating field-welded joints by applying polymer material in the form of a melt film or sheet on the surface of the pipe or pipe section from a moving die or nozzle. The melt polymer film or sheet is applied on the pipe from a die which is mounted on a carriage, which can be moved along the periphery of the pipe or pipe section. For smoothening and flattening of the polymer layer deposited on the surface and for improving the adherence of the layer to the pipe or pipe section there are provided levelling means such as a roller. The roller applies a force generally perpendicular to the polymer material layer. The surface of the roller is adjustable in the direction of the central axis to fit the surface profile of the pipe or pipe section at the part which is to be coated. The roller may also be mounted on the same movable carriage as the polymer deposition means of the apparatus, e.g. an extrusion nozzle or flat die.
WO 01/32316 discloses another pipe coating apparatus for coating girth welds of pipelines, which is equipped with spray coating means.
A perfectly coated and installed pipeline should be protected all over its length with equal quality coatings, including factory coating and field-joint coatings. Using most of the commercial coating solutions today this cannot be achieved. Some systems (e.g. injection moulding and machine taping) give relatively good protection but due to their technical solutions they are not suitable to be used afield and/or cannot be used for a wide register of different sizes of pipes.
When coating field-joints using the abovementioned melt-film technique, it is essential to apply a sufficient amount of polymer and to carefully follow the rules of welding plastic pipes, i.e. time, temperature and pressure. To avoid sagging of the polymer film, heavy pressure is applied thereon by the roller. However, this easily reduces the thickness of the film on the weld-seam area in an undesired way. Another problem is that the roller stats to glide against the pipe and pushes away the polymer coating, thus reducing the quality of the coating. These effects are illustrated in FIGS. 1A and 1B. FIG. 1A shows a coating arrangement according to WO 2008/132279. The pipe is denoted with numeral 10. Polymer is applied onto the pipe from application die 12 and levelled using a roller 13. Thus, a polymer layer 16 is formed. However, the molten polymer builds up in front of the roller, see build-up 16′, resulting in uneven and inconsistent polymer layer. Another problem associated with prior art is the sagging of polymer between 4 o'clock and 8 o'click directions, i.e. a thicker polymer layer at the lower part of the pipe, see sagging 16″ in FIG. 1B.
Thus, conventional methods and apparatuses are not optimal at least for applying relatively thick layers of polymer, in particular polyolefin (PO) or modified polyolefin.